(1) Field of the Invention
The present invention relates generally on demodulators for modulated RF signals, mobile communications devices comprising such a demodulator as well as the method for demodulating modulated RF signals on the basis of a four port structure. It is particularly suitable for the demodulation of signals having a finite number of different magnitude states, such as f.e. (n)QAM.
(2) Description of Related Art
six-port receiver is known acting in the direct conversion manner and allowing conversion from mm-wave range and microwave range directly to the base band. At the same time a classic I/Q-demodulation chip (digital or analogue) can be avoided. By using suitable calibration procedures the influences of the non-ideal passive RF-components including manufacturing tolerances can be minimised. The six-port receiver detects the relative phase and relative magnitude of two incoming RF-signals. The circuitry of the six-port receiver is realised using only passive components in combination with power sensors for the detection of the relative phase and the relative magnitude of the RF-signals. An important feature of six-port receivers is that fabrication tolerances can be calibrated, which inherently allows low-cost production.
In Bossisio, Wu xe2x80x9cA six-port direct digital millimeter wave receiverxe2x80x9d, Digest of 1994 IEEE MTT Symposium, vol. 3, page 1659-1662, San Diego, May 1994, a structure for a six-port receiver is proposed.
The six-port technique has been known for its ability to accurately measure the scattering parameters, both amplitude and phase, of microwave networks. Instead of using heterodyne receivers a six-port receiver accomplishes direct measurements at microwave and mm-wave frequencies by extracting power levels at at least three and particularly four of the 6 ports. The imperfections of the hardware can be readily eliminated by an appropriate calibration procedure. Six-port junction receivers consist of passive microwave components such as directional couplers and power dividers as well as diode detectors. The circuit can be easily integrated as MHMIC or MMIC. The known receiver performs direct phase/amplitude demodulation at microwave and mm-wave frequencies.
By performing a calibration procedure the hardware imperfections can be readily eliminated. This significantly eases the requirement of the hardware implementation and enables the six-port receiver to operate over a wide band up to mm-wave frequencies.
According to the above cited document of Bossisio et. al. a six-port receiver concept with power dividers and 90 degrees hybrid circuits realized in distributed technology is used. The application of that known structure lies mainly in the frequency bands above 10 GHz, however, it suffers from an insufficient band width of the operation due to the inherently frequency selective nature of the 90 degrees hybrid circuits.
FIG. 11 shows the structure of a six-port receiver known from Bossisio, Wu xe2x80x9cA six-port direct digital millimeter wave receiverxe2x80x9d, Digest of 1994 IEEE MTT Symposium, vol. 3, page 1659-1662, San Diego, May 1994.
The incoming digitally modulated RF-signal is compared with the output of a digital controlled local oscillator 218. Carrier recovery is first performed. The DSP-unit 217 detects-the frequency difference of the signals and then controls the local oscillator 218 to track the incoming signal. Once the carrier is recovered the instantaneous phase of the received signal is detected and decoded so as to recover the original modulated data. The maximum data transmission rate is determined mainly by the sampling rate of the A/D-converters 216 and the processing speed of DSP-unit 217.
From D. Maurin, Y. Xu, B. Huyart, K. Wu,M. Cuhaci, R. Bossisio xe2x80x9cCPW Millimeter-Wave Six-Port Reflectometers using MHMIC and MMIC technologiesxe2x80x9d, European Microwave Conference 1994, pp. 911-915, a wide-band topology for reflectometer used is known which is based on a distributing element approach featuring coplanar wave guide applications in the frequency range from 11 to 25 GHz.
From V. Bilik, et al. xe2x80x9cA new extremely wideband lumped six-port reflectometerxe2x80x9d European Microwave Conference 1991, pp. 1473-1477 and the idea of using Wheatstone Bridges and resistive structures for reflectometer applications is known.
From j:Li, G. Bossisio, K. Wu, xe2x80x9cDual tone Calibration of Six-Port Junction and its application to the six-port direct digital receiverxe2x80x9d, IEEE Transactions on Microwave Theory and Techniques, vol. 40, January 1996 a six-port reflectometer topology based on four 3 dB hybrid circuits, power dividers and attenuators is known.
From U.S. Pat. No. 5,498,969 an asymmetrical topology for a reflectometer structure featuring matched detectors and one unmatched detector is known.
From U.S. Pat. No. 4,521,728 with the title xe2x80x9cMethod and six-port network for use in determining complex reflection coefficients of microwave networksxe2x80x9d a reflectometer six-port topology is known comprising two different quadrate hybrids, phase shifter, two power dividers and one directional coupler for which the realization by a microstrip line technology is disclosed.
From EP-A-0 805 561 a method for implementing a direct conversion receiver with a six-port junction is known. According to this known technique, modulated transmitted modulation is received by a direct conversion receiver which comprises a six-port junction. The demodulation is carried out analogically.
From EP-A- 0 841 756 a correlator circuit for a six-port receiver is known. In this correlator circuit the received signal is summed up with a local oscillator signal at various phase angles, wherein the phase rotation between the local oscillator and RF signals is carried out separately from the summing of the correlator outputs.
In the following a four-port junction device (N=4) as a first example for the N-port junction technology will be explained with reference to FIG. 4. Such a four-port receiver is known from the post-published application PCT/EP 98/08329 in the name of SONY INTERNATIONAL (EUROPE) GMBH.
FIG. 10 shows the use of said known four-port junction device in a I/Q demodulator or QPSK demodulator. A signal is received by means of an antenna 426 and is then either supplied directly to a bandpass filter 428 or first downconverted optionally in a first stage downconverter 427. The output signal of the bandpass filter 428 is amplified by a gain controlled LNA block 429. The gain of the gain controlled LNA block 429 is controlled by a control unit 430. The amplified output signal of the gain controlled LNA block 429 is fed to the RF input 404 of the four-port junction device 401.
A RF switch 451 is connected to the second RF input port 405 of the four-port junction device 401. Depending on the switching position of the RF switch 451 the RF input port 405 of the four-port junction device 401 is either connected to ground potential by means of a resistor 450 with a resistance value of 50 xcexa9 (impedance matching) or a RF output signal of a local oscillator 420 is fed to the RF input 405 of the fourxe2x80x94port junction device 401. The frequency and the phase of the local oscillator 420 is also controlled by the control unit 430. Furthermore, the control unit 430 controls the switching effected by the RF switch 451.
The four-port junction device 401 comprises a first passive three-port structure and a second passive three-port structure. The first and the second passive three-port structure are connected with each other by means of a phase shifter. At a RF input port of the first passive three-port structure a RF signal to be processed is supplied. The first passive three-port structure has an output connected to a power sensor P1.
The second passive three-port structure of the four-port junction device according to the embodiment has a RF input port to which a second RF signal is fed wherein the second RF signal can e.g. originate from a local oscillator. The second passive three-port structure has an output port connected to a second power sensor P2.
In view of the above cited prior art it is the object of the present invention to propose a technique to demodulate modulated RF signals on the basis of a four port receiver structure with less complexity.
Thereby it is the central idea of the present invention to simplify the structure significantly by adapting it for the targeted application, i.e. the targeted modulation scheme used. To the approach according to the present invention is particularly well adapted for nQAM modulated RF signals.
According to the present invention therefore a demodulator for modulated RF signals comprising a passive four port structure (four port receiver structure) is provided. The passive four port structure has a first input for supplying a modulated RF signal to be demodulated. A second input of the four port structure is supplied with the RF signal from a local oscillator. Two output terminals are respectively connected to a power sensor. Thereby the RF signal from the local oscillator is supplied to the second input of the four port structure without being switched by a RF switch. In comparison to the post-published prior art the structure of the demodulator therefore is simplified by the omission of the RF switch (which is connected between the local oscillator and the second input of the four port structure according to the post-published prior art).
An isolation circuitry can be connected to the first input of the passive four port structure for providing a RF/LO isolation.
The four port structure can comprise two three port structures connected with each other by means of a phase shifter.
The four port structure can be implemented by resistive elements.
The low pass filter can be attached to the output of the power sensors.
The output of the low pass filter can be supplied to RID converter and furthermore to a digital processing unit.
At least one of the output signals of the low pass filter can be supplied to an analog averaging unit averaging a plurality of symbols and the output of the analog averaging unit is supplied to an analog processing unit together with the output of low pass filter.
According to the present invention furthermore a mobile communications device comprising a demodulator as said forth above is proposed.
According to another aspect of the present invention a method for demodulating modulated RF signals is proposed. A modulated RF signal to be demodulated is supplied to a first input of a passive four port (receiver) structure. A RF signal from a local oscillator is supplied to a second input of the passive four port structure. Two outputs of the four port structure are supplied to power sensors. The RF signal from the local oscillator thereby is supplied to the second input of the four port structure without RF switching.
The RF signal from the local oscillator can be directly supplied to the second input of the four port structure. The RF input (first input) of the passive four port structure can be isolated to the LO (second input) of the passive four port structure.
The output of the low pass filter step can be A/D converted and then digitally processed.
At least one of the outputs of the low pass filter step be averaged in an analog manner over a plurality of symbols and the output of the analog averaging step can be analog processed together with the output of the low pass filter step.